FUNCTIONAL EXAMINATION OF THE EAR
octave steps, i.e. 125, 250, 500, 1000, 2000, 4000 and 8000 cycles per second.
In some instruments half-octave steps are available to test intermediate
frequencies. At each frequency the intensity of the sound can be raised in
5-decibel steps above the normal threshold of hearing until the sound can
just be heard. In a deaf ear, the amount that the intensity of the sound has to
be raised above the normal level is a measure of the degree of deafness at that
frequency. By measuring the threshold of hearing at each frequency, an
accurate measure of the degree of hearing loss in each ear is obtained.
Instead of using earphones to measure air-conducted sound, a receiver can be
applied directly to the mastoid process and sound is transmitted to the
cochlea by bone conduction. The sound-conduction apparatus is by-passed
so that the threshold of bone conduction is a measure of inner ear function.
As with the air conduction, measurement of bone conduction is expressed in
decibels of hearing loss below the normal. The results are charted as audio-
grams (F/>. 147).
In audiometry it is important to eliminate the possibility that the test
sound is being heard in the opposite ear. The audiometer provides a masking
tone which may be played into the opposite ear. Masking must be applied to
the better ear when testing the deafer ear if the difference in threshold is
found to be more than 30 decibels. When testing the bone-conduction
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Fig. 147. Pure-tone audiograms. A, Normal; B, Conduction (middle-ear) deafness; C, Perceptive (nerve)
deafness. Air conduction: Right = O; left = x . Bone conduction: Right = [; left = ].